Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-08-05
2001-08-14
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S439000, C455S442000
Reexamination Certificate
active
06275703
ABSTRACT:
This invention relates to a method and system of controlling handover in a mobile communication network.
In a mobile communication network, each mobile station in dedicated mode communicates with a base station via a traffic channel which is determined depending on the environment within which the mobile station is located, the signal levels between the mobile station and the base station, interference of the signal transmitted between the mobile station and the base station and other factors which affect the signals which are transmitted between the mobile station and the base station in handling a call. The term traffic channel is used herein to denote a channel set up for communication of signals between a mobile station and a base station. In a standard GSM TDMA system, a traffic channel is defined by a particular frequency for signal transmission and a particular time slot allocation or in case of frequency hopping by a hopping sequence. In some, more sophisticated, systems it may alternatively or additionally be defined by the radiation direction of a signal as in so-called SDMA systems.
In the following discussion of handover techniques it will be appreciated that it is generally the frequency of a traffic channel which is under discussion or in case of frequency hopping the set of frequencies allocated to the mobile station called mobile allocation. That is, the terms frequency, frequency channel or channel are used to denote a frequency used to carry traffic.
There are several reasons why a traffic channel may need to be altered during the progress of a call. As the mobile station moves, its relationship in signalling terms with the base station is constantly being altered and with it the signal parameters of the communication between the mobile station and the base station alter. According to the existing handover techniques, one or more of these signal parameters is monitored to detect a trigger condition for handover and, when that trigger condition is detected, handover is implemented to another traffic channel. These signal parameters can include the following:
i) the quality of the signal on the up-link or down-link,
ii) the level of the signal on the up-link or down-link,
iii) timing advance information (which is a measure of how far away the mobile station is from the base station with which it is presently communicating),
iv) traffic levels in the area, and
v) power budget considerations.
The traffic channel for handover is selected by virtue of various different handover algorithms which are dependent on the communication environment. If after the handover the quality of the call is improved, the handover decision is good. However, there are many situations where the handover decision is bad, and the call is either dropped or the quality deteriorates. The major reason for this is that the handover decision is based only on signal level information from the handover candidates.
It is thus an object of the present invention to improve the success of handover decisions so that a larger majority of calls maintain good quality after being handed over.
According to one aspect of the present invention there is provided a method of controlling handover in a mobile communication network wherein an instant traffic channel providing communication between a mobile station and a base station is monitored to detect a trigger condition for handover to a different traffic channel, the different traffic channel being selected from a plurality of handover alternatives, the selection of one of said plurality of handover alternatives as said different traffic channel being based on prestored probability indicators associated respectively with each handover alternative, each probability indicator indicating a likely probability of success if handover is effected to that handover alternative.
By storing probability indicators with each handover alternative, it is possible to effect a handover decision which uses the handover alternative having the highest probability of success in that particular environment.
In the described embodiment, such a plurality of handover alternatives and associated probability indicators are stored for each of a set of cell patterns, each cell pattern comprising a group of neighbouring cells in the network. These cell patterns are referred to herein as server patterns and can comprise a group of up to four cells including the server cell and the three neighbouring cells having the best reported signal levels. The handover alternatives and associated probability indicators are referred to herein as a server pattern probability vector (SPPV). In the preferred embodiment therefore there is a server pattern probability vector associated with each server pattern.
According to another aspect of the present invention there is provided a method of effecting handover in a mobile communication network comprising the steps of:
monitoring a first traffic channel providing communication between a mobile station and a base station to detect a trigger condition for handover;
after said trigger condition has been detected, selecting from a plurality of handover alternatives a second traffic channel to provide said communication, and effecting handover to said second traffic channel;
during a monitoring period commencing at detection of the trigger condition, monitoring the first and second traffic channels and the remaining handover alternatives; and
storing channel data collected during the monitoring period for use in subsequent handovers.
The feature of this aspect of the invention can be combined with the feature of the first aspect of the invention by using the channel data collected during the monitoring period on the instant traffic channel and the remaining handover alternatives to determine and update the probability indicators.
The monitoring period can commence at a registration point at which the trigger condition is detected. It can terminate at an evaluation point where the success or failure of the selected traffic channel is decided, the success or failure being taken into account to determine and update the probability indicator.
This aspect of the invention also provides a system for effecting handover in a mobile communication network, the system comprising:
means for monitoring a first traffic channel providing communication between a mobile station and a base station to detect a trigger condition for handover;
means for selecting from a plurality of handover alternatives a second traffic channel to provide said communication after said trigger communication has been detected;
switching means for effecting handover from the first traffic channel to the second traffic channel;
means for monitoring the first and second traffic channels and the remaining handover alternatives during a monitoring period commencing at detection of the trigger condition; and
a store holding channel data collected during the monitoring period for use in subsequent handovers.
According to another aspect of the present invention there is provided a system for controlling handover in a mobile communication network, the system comprising:
means for monitoring an instant traffic channel providing communication between a mobile station and a base station to detect a trigger condition for handover to a different traffic channel;
means for selecting the different traffic channel from a plurality of handover alternatives;
a store holding said plurality of handover alternatives associated respectively with prestored probability indicators, each probability indicator indicating a likely probability of success if handover is effected to that handover alternative, wherein the selecting means selects said different traffic channel based on said probability indicators.
In accordance with a further aspect of the present invention there is provided a mobile communication network comprising a plurality of base stations for defining cells in the network and at least one base station controller for controlling said base stations, the base station controller including a system for controlli
Bost Dwayne
Merchant & Gould P.C.
Nokia Telecommunications Oy
Trinh Sonny
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